An Intro to Constructors in C#

This is an article on Constructors in C#, for the beginner level programmers. It covers simple constructors, constructors overloading, behaviour of constructors in inheritance, constructor chaining and static constructors. At the end, it contains the general FAQs about constructors.

Introduction

Broadly speaking, a constructor is a method in the class which gets executed when its object is created. Usually, we put the initialization code in the constructor. Writing a constructor in the class is damn simple, have a look at the following sample:

publicclass mySampleClass
{
public mySampleClass()
{
// This is the constructor method.
}
// rest of the class members goes here.
}

When the object of this class is instantiated, this constructor will be executed. Something like this:

mySampleClass obj = new mySampleClass()
// At this time the code in the constructor will // be executed

Constructor Overloading

C# supports overloading of constructors, that means, we can have constructors with different sets of parameters. So, our class can be like this:

publicclass mySampleClass
{
public mySampleClass()
{
// This is the no parameter constructor method.// First Constructor
}
public mySampleClass(int Age)
{
// This is the constructor with one parameter.// Second Constructor
}
public mySampleClass(int Age, string Name)
{
// This is the constructor with two parameters.// Third Constructor
}
// rest of the class members goes here.
}

Well, note here that call to the constructor now depends on the way you instantiate the object. For example:

mySampleClass obj = new mySampleClass()
// At this time the code of no parameter // constructor (First Constructor)will be executed
mySampleClass obj = new mySampleClass(12)
// At this time the code of one parameter // constructor(Second Constructor)will be // executed.

The call to the constructors is completely governed by the rules of overloading here.

Calling Constructor from another Constructor

You can always make a call to one constructor from within another. Say, for example:

publicclass mySampleClass
{
public mySampleClass(): this(10)
{
// This is the no parameter constructor method.// First Constructor
}
public mySampleClass(int Age)
{
// This is the constructor with one parameter.// Second Constructor
}
}

Very first of all, let us see what is this syntax:

public mySampleClass(): this(10)

Here, this refers to same class, so when we say this(10), we actually mean execute the public mySampleClass(int Age) method. The above way of calling the method is called initializer. We can have at the most one initializer in this way in the method.

Another thing which we must know is the execution sequence i.e., which method will be executed when. Here, if I instantiate the object as:

mySampleClass obj = new mySampleClass()

Then the code of public mySampleClass(int Age) will be executed before the code of mySampleClass(). So, practically the definition of the method:

public mySampleClass(): this(10)
{
// This is the no parameter constructor method.// First Constructor
}

is equivalent to:

public mySampleClass()
{
mySampleClass(10)
// This is the no parameter constructor method.// First Constructor
}

Note: Above (just above this line) code is mentioned there for pure analogy and will not compile. The intention here is to tell the flow of execution if initializers are used.

We cannot make an explicit call to the constructors in C#, treating them as if any simple method, for example: statement mySampleClass(10) in the above code will not work. The only way you can call one constructor from another is through initializers.

For the VB.NET programmers: you can make the call to another constructor of the same class by the syntax Me.New(param list), but it should be the first line of your calling constructor method. So ultimately, the code of the called constructor runs prior to the code of the calling constructor, which is same as initializers here.

Note that only this and base (we will see it further) keywords are allowed in initializers, other method calls will raise an error.

This is sometimes called Constructor chaining.

Huff… Simple thing made tough, but this is how it is. Anyway, let us proceed further.

Note: If we do not provide initializer referring to the base class constructor then it executes the no parameter constructor of the base class.

Note one thing here: we are not making any explicit call to the constructor of base class neither by initializer nor by the base keyword, but it is still executing. This is the normal behavior of the constructor.

If I create an object of the derived class as:

myDerivedClass obj = new myDerivedClass(15)

Then the sequence of execution will be:

public myBaseClass(int Age) method

and then public myDerivedClass(int Age) method

Here, the new keyword base has come into picture. This refers to the base class of the current class. So, here it refers to the myBaseClass. And base(10) refers to the call to myBaseClass(int Age) method.

Also note the usage of Age variable in the syntax: public myDerivedClass(int Age):base(Age). [Understanding it is left to the reader].

Private Constructors

Private constructors, the constructors with the "private" access modifier, are a bit special case. It is because we can neither create the object of the class, nor can we inherit the class with only private constructors. But yes, we can have the set of public constructors along with the private constructors in the class and the public constructors can access the private constructors from within the class through constructor chaining.

will raise an error : 'Constructors.MyClass.MyClass()' is inaccessible due to its protection level

It is possible to have the class with only the private constructors. But yes as I said, such class can neither be instantiated nor be inherited. If we try to inherit the class with only private constructors then we will get the same error as above. Also recall, once you provide constructor from your side the compiler will not add the no-parameter public constructor to your class.

Well, one of the usage scenarios of such class could be – when you have only static members in the class and you don't need to instantiate it.

Static Constructors

This is a new concept introduced in C#. By new here, I mean that it was not available for the C++ developers. This is a special constructor and gets called before the first object is created of the class. The time of execution cannot be determined, but it is definitely before the first object creation - could be at the time of loading the assembly.

The syntax of writing the static constructors is also damn simple. Here it is:

Ok fine, all the above points are fine, but why is it like that? Let us go step by step here.

Firstly, the call to the static method is made by the CLR and not by the object, so we do not need to have the access modifier to it.

Secondly, it is going to be called by CLR, who can pass the parameters to it, if required. So we cannot have parameterized static constructor.

Thirdly, non-static members in the class are specific to the object instance. So static constructor, if allowed to work on non-static members, will reflect the changes in all the object instances, which is impractical. So static constructor can access only static members of the class.

Fourthly, overloading needs the two methods to be different in terms of methods definition, which you cannot do with Static Constructors, so you can have at the most one static constructor in the class.

This is perfectly valid, though doesn't seem to be in accordance with overloading concepts. But why? Because the time of execution of the two methods are different. One is at the time of loading the assembly and one is at the time of object creation.

Constructors FAQs

Is the constructor mandatory for a class?

Yes, it is mandatory to have the constructor in the class and that too should be accessible for the object i.e., it should have a proper access modifier. Say, for example, we have only private constructor(s) in the class and if we are interested in instantiating the class, i.e., want to create an object of the class, then having only private constructor will not be sufficient and in fact it will raise an error. So, proper access modifies should be provided to the constructors.

What if I do not write the constructor?

In such case, the compiler will try to supply the no parameter constructor for your class, behind the scene. Compiler will attempt this only if you do not write the constructor for the class. If you provide any constructor (with or without parameters), then compiler will not make any such attempt.

What if I have the constructor public myDerivedClass(), but not the public myBaseClass()?

It will raise an error. If either the no parameter constructor is absent or it is in-accessible (say it is private), it will raise an error. You will have to take the precaution here.

Can we access static members from the non-static (normal) constructors?

Yes, we can. There is no such restriction on non-static constructors. But there is one on static constructors that it can access only static members.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

here i have just written an explicit constructor..
As per my Knowledge, Constructor is called when ever the class instance is created;
but i had a Debate with my friend that, He was saying like, First constructor gets called then instance will be created then.
Can you help me?
Which one is correct?

First constructor is called then instance is created as that is the task of constructor to calculate the amount of space required in the memory by calculating the amount of space by all the fields. Once it is done with the summing up the bytes required it finalizes the space in memory and creates instance.

Then the sequence of execution will be:
1. public myBaseClass(int Age) method
2. and then public myDerivedClass(int Age) method

Is the sequence of execution correct for above line of code.
I think, The sequence will will be like:
1. public myBaseClass() method
2. and then public myDerivedClass(int Age) method
Because of its behaviour that the constructior with zero argument of base class will be executed everytime any constructor of child class having N argument is invoked.

publicclass mySampleClass
{
public mySampleClass(): this(10)
{
// This is the no parameter constructor method.// First Constructor
}
public mySampleClass(int Age)
{
// This is the constructor with one parameter.// Second Constructor
}
}

and

public mySampleClass(): this(10)

How did the the second constructor got called first?

Whattup,. let's do this right,.
http://www.gundamwarheads.wordpress.com/

Hi,
It's ok that static constructors are not supposed to be called explicitly.
Then what does the statement "These constructors have implicit private access" (taken from a technical resourse), mean?
As far as i can deduce from the supposed meaning of the Private access modifier, one cannot access it beyond the scope of the containing class.
I created a couple of classes containing an instance constructor and a static constructor, respectively. As the context suggests, the instanciation of the first class can be achieved within the scope of the latter class, But when considered vice-versa, the result is same, i.e, the constructor being "implicitly private in nature", is easily accessed, contradicting the privacy!!!
I did this (calling the static constructor), by creating an object of that class(error free compilation!) , which although is against the conventional calling syntax for a static member, is providing this effect.

I'm just trying to follow the rules and found this discrepancy. Kindly respond with my query and rectify me if I am wrong anywhere.The implemented code is included below for the reference

Pretty good article. I suggest you to elaborate a little on private constructor. My teammate read this article and understood that he cannot create object of the class which has a private constructor. But a private constructor class could be instantiated in the class itself. If we refer to singleton where we have private constructor and instantiation of the class happens in the class itself. The context might be different. if you want to create the object(instantiation) outside the class, it is not possible.

@Chathura, Can you please elaborate on what those problematic facts are... I am started trying to understand the language with a lot more detail and any issues you see with the article will help me to dig further and understand them.. thank you.